The invention concerns a process for the control of exposure in a photographic color copying apparatus.
To determine the optimum exposure times and quantities of copying light in a photographic color copying apparatus the color permeabilities or color densities of a copy master in the three basic colors are determined. It is necessary in the process to adapt the spectral sensitivity of the measuring apparatus as accurately as possible to the spectral sensitivity of the copy material used. This requirement is the result of the experience when deviations of the spectral sensitivities of the measuring layout and the copy material occur, an increase in the density of a color of the copy master does not lead to a corresponding change in the quantity of copying light and that color falsification may therefore result. This is discussed for example in the publication by R. W. G. Hunt, "The Reproduction of Color", p. 284.
From U.S. Pat. No. 4,589,766 a photographic color copying apparatus is known, wherein an attempt is made to satisfy this requirement by means of special optical measuring filters. The disadvantage of this method is apparent when different copy materials are used, for example with matte or glossy surfaces. For every change of copy material, new measuring filters adapted to the altered spectral sensitivities of the copy material must be used, which considerably increases costs and requires additional setup times. Furthermore, in view of the extreme precision required, these measuring filters are very difficult to produce and are therefore expensive. Another expense is caused by the logistic management of the different measuring filters adapted to the spectral sensitivities of the copy material. In addition, the choice of a certain set of measuring filters in most cases restricts the user to a certain type of copy material of a certain manufacturer.